In most spontaneously ovulating mammals the corpora lutea formed at ovulation secrete progesterone for 2 weeks. On the other hand, the corpora lutea formed at ovulation in the rat secrete progesterone for only a few hours during the estrous cycle. However, a prolonged luteal phase or pseudopregnancy of approximately 2 weeks can be induced in the rat by mechanical or electrical stimulation of the uterine cervix on proestrus. Such stimulation leads to 10 days of daily diurnal and nocturnal surges of prolactin secretion, the hormone responsible for maintenance of the corpus luteum. Since these prolactin surges can be induced at an attenuated level in ovariectomized rats for only 6 days, it has been proposed that the information provided by cervical stimulation is stored within the hypothalamus and induces minimal prolactin surges for the initial 6 days. Thus, reinforcement by ovarian steroids may be required for initiation of prolactin surges of normal magnitude and their persistence beyond day 6. One objective of the proposed research will be to determine the ovarian steroid milieu responsible for initiation, maintenance beyond day 6 and termination after day 10 of normal nocturnal and diurnal surges of prolactin secretion induced by cervical stimulation. Ovarian steroid secretion will be simulated in ovariectomized rats with Silastic implants and serum concentrations of prolactin, estrogen and progesterone will be determined by radioimmunoassay. In addition, through appropriate lesions and stimulations, the hypothalamic centers responsible for translating the information provided by cervical stimulation and lighting periodicity into prolactin surges will be determined.